U.S. Democratic presidential candidate Hillary Clinton addresses a joint gathering of the National Association of Black Journalists and the National Association of Hispanic Journalists in Washington August 5, 2016. REUTERS/James Lawler Duggan

NEW YORK (Reuters) - Democratic presidential nominee Hillary Clinton's lead over Republican rival Donald Trump narrowed to less than 3 percentage points, according to a Reuters/Ipsos opinion poll released on Friday, down from nearly eight points on Monday.

About 42 percent of likely voters favored Clinton, to Trump's 39 percent, according to the July 31-Aug. 4 online poll of 1,154 likely voters. The poll had a credibility interval of plus or minus 3 percentage points, meaning that the results suggest the race is roughly even.

Among registered voters over the same period, Clinton held a lead of five percentage points, down from eight percentage points on Monday, according to the poll.

The reasons behind the shift were unclear.

Clinton had pulled well ahead of Trump on the heels of the Democratic National Convention last week, where she became the first woman to accept the U.S. presidential nomination from a major political party.

Since then, Trump has engaged in a days-long feud with the family of an American soldier killed in Iraq and squabbled with the Republican leadership over his comments and leadership turmoil within his campaign.

Trump, in recent days, however, has sought to refocus. On Friday he announced his economic policy advisory team, said he would deliver an economic policy speech early next week, and was expected to endorse U.S. House of Representatives Speaker Paul Ryan, the top U.S. elected Republican, who is seeking his 10th term in Congress.

An average of polls aggregated by Real Clear Politics showed Clinton ahead of Trump by 6.8 percentage points on Friday, up from 3.9 on Aug. 1.